Loss of neurons in the hilus of the dentate gyrus is one of the most consistent morphological findings in humans with temporal lobe epilepsy and related models of acquired epilepsy. Two major groups of hilar neurons are frequently damaged, GABAergic somatostatin interneurons and glutamatergic mossy cells. Despite years of interest in hilar neurons, important questions persist. Why are the neurons so vulnerable to excitotoxic damage? How could they be protected? What are the functional effects of both loss and preservation of subgroups of these neurons? A set of new technologies, reagents and mice will now allow probing these questions in exciting new ways. The proposed studies will use mice with Cre-recombinase expression in these neurons to selectively manipulate them through Cre-activated viral gene expression. A combination of light and electron microscopic immunohistochemical methods will be used to evaluate the changes, and functional correlates will be determined with electrophysiological methods. The broad goals are to determine if increasing tonic GABAergic inhibition in these neurons could be neuroprotective and, in a separate set of studies, to map the functional circuits that are activated following manipulation o either mossy cells or somatostatin neurons in normal and seizure-prone animals in vivo. Specific Aim 1 will test the hypothesis that both groups of hilar neurons lack substantial expression of the subunit of the GABAA receptor (GABAAR) and have low levels of tonic inhibition. Specific Aim 2 will test the hypothesis that expressing an exogenous GABAAR subunit, which is normally involved in tonic inhibition in the cerebellum, will lead to the formatin of functional GABAA receptors and increase tonic inhibition in both groups of hilar neurons. Specific Aim 3 will test the hypothesis that increasing tonic inhibition in hilar neurons will protct them from damage following status epilepticus. Specific Aim 4 will examine the functional circuitry of hilar neurons in vivo by using optogenetic methods to manipulate the neurons selectively and then identifying the activated neurons by Fos labeling. The patterns of activation in normal and seizure-prone animals will be compared to test the hypotheses that stimulating remaining mossy cells or silencing remaining somatostatin interneurons in vivo will lead to increased granule cell activation in the epileptic mice. By allowing selective manipulation of hila neurons in the intact brain, these studies will provide unique views of their function within norma and altered neuronal circuits. PUBLIC HEALTH RELEVANCE: This proposed research is relevant to public health because it will provide new information about neurons that are frequently damaged in epilepsy, ischemia and traumatic brain injury. Loss of specific groups of neurons in the hippocampus is one of the most consistent findings in acquired epilepsy in humans and related animal models. These studies will evaluate the effects of increasing inhibition in these vulnerable neurons with the goa of finding new ways to protect the neurons from seizure-induced damage and will also identify the functional effects of manipulating these neurons in the intact brain.